sfxge: add function to query link control privilege
Make link control privilege visible to OS driver to guard updates to
flow control and PHY advertised capabilities.
Submitted by: Richard Houldsworth <rhouldsworth at solarflare.com>
Sponsored by: Solarflare Communications, Inc.
MFC after: 2 days
Differential Revision: https://reviews.freebsd.org/D4330